The present invention generally relates to the fabrication of semiconductor devices from substrates, and relates in particular to the use of strained silicon (Si) heterostructure substrates in forming devices such as transistors for example for high-performance CMOS integrated circuit products.
As microelectronic devices require faster operating speeds and increased computing power, the need exists for transistor circuits to provide a greater complexity of transistors in a smaller amount of circuit real estate. Such microelectronic devices include, for example, microprocessors, ASICs, embedded controllers, and FPGAs. Each microelectronic device consists of millions of transistors, such as metal oxide semiconductor field-effect transistors (MOSFETs), that are designed to provide control over both the directional flow of electrons and the speed at which the electrons move through the circuits.
MOSFETs are conventionally fabricated on Si substrates, which are the basic starting substrates on which semiconductor circuits are built. In order to create a MOSFET device on a Si substrate, a very thin layer of insulator is thermally grown or deposited on the Si substrate followed by a polysilicon gate electrode definition to create a MOSFET device. Typically this insulator is SiO2, or SiO2 with a significant fraction of nitrogen, and so the insulator is typically referred to as the gate oxide. The thickness of the gate oxide can determine the threshold voltage that must be applied to the gate of a MOSFET to turn on the MOSFET device. The gate oxide thickness is used to define the MOSFET application. For example, high-performance microprocessors have core logic devices with ultra-thin (e.g., 10–20 Å) gate oxides and input/output devices with thicker gate oxides (e.g., 20–100 Å). As the operational speed of electrical systems has increased, it has become necessary to have MOSFET devices with different gate oxide thicknesses on the same chip.
Conventional oxidation techniques for thermally growing oxide layers on a Si substrate typically involve the consumption of a significant portion of the Si substrate. For example, the amount of Si substrate that is lost in the oxidation process may be approximately one half of the thickness of the resulting thermally grown oxide layer.
Strained silicon heterostructures provide semiconductor devices with enhanced electron mobility and therefore speed. For example, see U.S. Pat. No. 5,442,205. Strained silicon heterostructure substrates typically include a relatively thin (e.g., less than 250 Å) strained silicon layer that may be used as the channel in a MOSFET device. If the layer of strained silicon is grown too thick, misfit dislocation defects will occur in the layer, compromising the yield (percentage of functional devices) when MOSFET circuits are fabricated on the substrate. In particular, at a critical thickness, dislocations are favored for strain relief of the epitaxial film over continued accumulation of strain energy. The critical thickness is a function of the lattice mismatch between the epitaxial film and substrate, as well as the materials properties of both the epitaxial layer and the substrate. It is this critical thickness that may limit the useful strained silicon film thickness to less than, e.g., 250 Å.
If too much of the strained silicon layer is consumed in the oxidation process, then the layer will be too thin to obtain the benefits of the enhanced electron mobility. The minimum strained silicon film thickness required for significant mobility enhancement is approximately 50 Å. Conventional methods of forming multiple gate oxides do not work well on a strained Si substrate since the strained Si cap layer may be too thin to support the formation of both thick and thin gate oxides. This is particularly the case since during a typical MOSFET fabrication process, there is much additional strained Si consumption due to various processing steps (cleans, thermal oxidations, anneals).
There is a need, therefore, for a method of forming strained silicon heterostructure substrates having a plurality of gate oxide thicknesses without sacrificing the enhance electron mobility of the substrates.